would there be any problem of upsizing tires on my 2003 ford f150 reg cab from 255/70/r16 to a 265/70/r16, would there be plenty of clearance without cutting turn radius or hitting fenders, or any problems with that size tire ?

 

Use this: https://tiresize.com/calculator/

and a measuring tape.

 

That's only one size larger than stock for your truck. Turn the front wheels lock to lock and check clearances. that was the stock size for my 2000 F150Scab4x4 but the 4x4s are raised up a couple inches. Larger tires will reduce fuel economy.